Crude oil is processed by a variety of means to produce the valuable light fractions such as gasoline, kerosene, and the like. Since these products are valuable commodities, it makes economic sense to try and convert as much of the oil as possible to these components. As much as 75% of a barrel of oil can be utilized in this way by a combination of distillation and catalytic cracking. In catalytic cracking, the heavy oil components which were left over from distillation are broken into smaller molecular weight components with the aid of a cracking catalyst. The catalysts are porous aluminum-silicate structures. At the end of the cracking process the material which was not broken down remains in the product. The sludge oil contains significant amounts of catalysts, called “fines” or “catfines” typically as very small particles (0.1 to 8 um) in size. The sludge oil is mixed with other feedstock to produce the heavy fuel oils used for large marine diesel engines. The presence of high (>80 ppm) levels of aluminum and silicon and other catalyst fines in the fuel can cause problems when the fuel is injected into the engine. The catalyst fines are very hard materials which can cause rapid wear on the engine components such as the cylinder, pistons and valves. For this reason, one standard dictates that all fuel must be prepared with a total combined Al+Si concentration of <80 ppm.
Fuel oil analysis often involves several specialized pieces of equipment operated by scientists in a laboratory. The current method used for measuring low concentrations of catalyst fines is an ISO method, ISO 10478: 1994. This standard method states that the amount of aluminum and silicon should be measured by using Inductively Coupled Plasma (ICP) spectroscopy analysis. ICP is a sensitive technique that is able to measure most elements down to the 1-10 ppb level in the sample introduced into the spectrometer. However, the ICP technique requires that the sample be in liquid form so that it can be sprayed into the plasma. This means that for solid samples or viscous liquids, some form of dissolving and dilution is required. Then, the detection limits in the original sample are 10-100 times higher, depending on the amount of dilution that has occurred. The major drawbacks of ICP for testing in the field (e.g., on-ship or dockside) are that such systems are relatively expensive instruments that use expensive consumables such as liquid Argon, and are not portable. In addition this method is time consuming and requires several analytical steps that require a skilled technician to carry out the procedure.
X-ray fluorescence (XRF) is a non-destructive technique used to analyze samples with a minimum of preparation. To make a compact XRF spectrometer, however, requires the use of an energy dispersive detector. This means that the detector collects and measures all energies simultaneously. The detector electronics separates out the different x-rays. The alternative is a “wavelength dispersive” system which separates the x-rays into different energies by using a crystal. The x-rays of each single energy are then counted sequentially.
The elements in the catalyst fines (Al and Si) are amongst the lightest elements that can routinely be measured at low levels with XRF. In a typical bench top XRF system, the detection limits for these elements are typically 500-1000 ppm, which is 100 times larger than is required for accurate quantification of Al and Si in fuel. Hence, there are several technological problems which must be overcome.
The fluorescent x-rays from Al and Si have relatively low energies of 1.486 keV and 1.740 keV respectively. These x-rays travel a relatively short distance through solids or air before being absorbed.
The yield of x-rays from elements is proportional to the atomic number Z. This means that for every x-ray absorbed by an atom, the chance of producing a fluorescent x-ray is low for Al and Si. The highest probability of producing a fluorescent x-ray occurs when the exciting x-ray is at just a slightly higher in energy than the energy to be emitted. This means that to best excite Al and Si with the same x-rays, an ideal energy of approximately 2 keV is needed. But, most x-ray tubes have a very low intensity output at such a low energy due to a relatively thick Be window (75 um) which cuts out much of the intensity below 2 keV and also because the standard anode materials used have no lines in this region of the spectrum.
In heavy fuels there is a further complication, in that the fuel contains greater than 0.5% sulfur. In some cases the sulfur content can be as high as 5-6%. In an energy dispersive XRF system, the amount of data that can be collected per second is limited by the x-ray detector. The presence of high levels of sulfur affects both the amount of Al and Si x-rays which can be collected and also changes the shape of the spectrum in the region where Al and Si are observed.
Moreover, XRF techniques for analyzing fuel oils have not yet been employed in the field because the laboratory equipment used for the analysis is not engineered for use outside of a laboratory.